The importance of energy harnessing and storage devices such as solar cells and batteries in this era of high gasoline prices hardly needs to be emphasized. Metal-air batteries combine a metal anode (similar to that used in conventional primary batteries) and an air gas-diffusion electrode (cathode) similar to that used in fuel cells. During operation the metal anode is electrochemically oxidized at the expense of the oxygen from air, which is reduced on the air-gas diffusion electrode (or air electrode, AE). AE possesses two distinct advantages over the conventional metal-oxide cathode in the primary batteries: infinite charge capacity and low weight independent of the capacity. The ampere-hour capacity of the metal-air battery is determined by the amount of the active metal anode, but not by the air electrode. Hence, the specific characteristics (such as Ah/kg and Ah/l) of the metal-air batteries are significantly higher than that of the other electrochemical energy storage systems. Among metal-air batteries, zinc-air batteries offers the highest specific energy (Wh/kg) and energy density (Wh/L) steady performance, and low-cost. Zinc-air batteries contain non-toxic materials and can operate as long as 10 to 14 hours compared to 2 to 4 hours for most rechargeable lithium-ion batteries and can be stored for long periods of time without losing their charge. The lightweight of zinc-air batteries leads to good power density (power per unit of weight or volume), which is very ideal for portable applications.
Zinc-air battery produces electricity by the electrochemical coupling of a reactive metallic zinc anode to an air cathode through a suitable aqueous electrolyte in a cell. During cell operation (discharge) oxygen adsorbed from surrounding air is reduced (OH−) using an oxygen reduction catalyst (ORR) at the cathode (referred to as air electrode, AE) while zinc metal (anode) is oxidized (Zn2+), providing a usable electric current flow through an external circuit connected between the anode and the cathode. Since Zn can be electrodeposited from aqueous electrolytes to replenish the anode, zinc-air batteries can be secondary as well as primary batteries.
At the anode:2Zn + 4OH−→ 2ZnO + 2H2O +E0 = −1.25 V4e−At the Cathode:O2 + 2H2O + 4e−→ 4OH−E0 = 0.40 VOverall reaction:2Zn + O2→ZnOE(OCV) = 1.65 V
The air-gas diffusion electrode (AE) has one surface exposed to the atmosphere and another surface exposed to the aqueous electrolyte of the cell and is the limiting component in the Zn-air battery cell performance. It is because oxygen reduction at the cathode poses substantial polarization loss, contributing to most of the voltage reduction in cell discharge. Therefore development of AE with facile electrochemical kinetics is of particular importance. Many factors have been established to affect the electrocatalytic performance of AE, (i) the carbonaceous material employed must be conductive, possesses large surface area and have sufficient sites to enhance oxygen reduction reaction (ORR), (ii) the electrocatalyst chosen requires efficient breaking of oxygen bonds, (iii) Internal pore structure of the AE should be optimized for proper channeling of electrolyte and gaseous reactant, and (iv) the hydrophobicity of the AE is tuned to minimize electrolyte flooding. Hence it is recognized that performance of the AE hinges immensely on its construction and the types of carbon materials and electrocatalyst employed.
Owing to the above factors the architectures of the commonly used AE are rather complex (FIG. 1A). In general, it is made up of a hydrophobic layer 3 (carbon) that is in contact with air and a hydrophilic layer 4 (carbon) that is in contact with the electrolyte 1. The hydrophobic layer 3 should be impermeable to the aqueous electrolyte but should be permeable to air. The hydrophilic layer 4 consists of single/multiple oxygen reduction catalyst. Both the layers comprises of a suitable polymer binder that binds the layers on to a conductive metal screen 5 (usually made of Nickel). The hydrophilic layer 4 consists of a physical mixture of carbon and suitable ORR catalyst (noble metals (Pt, Ag) or oxides (MnO2, perovskites)).
Present day Zinc air batteries consists of AE that are constructed by making a thick paste of active carbon, binder and pressing then against a nickel metal screen which acts as both the support and current collector. One side of the Nickel metal screen then consists of a hydrophobic layer (active carbon+binder) the other side consists of hydrophilic layer (active carbon, binder, catalyst).
Carbon nanotubes (CNTs) have attracted much attention in both fundamental and applied study owing to their high surface area, exceptional electrical, mechanical and thermal properties. Due to these properties, CNTs are considered as prospective catalyst support and replacement of active carbon in metal-air batteries and fuel cells. Owing to this, there are studies on the electrocatalytic oxygen reduction on carbon nanotubes with/without catalytic particles (such as Ag—MnO2, Pt, MnO2). It was reported in these studies that using CNT as catalyst support not only improved the conductivity of the AE but also significantly increased the active three-phase interfacial area for oxygen molecular reduction process. In the above studies, the AE were made in the conventional method of mixing CNT with binder and other additives and was placed on a nickel mesh screen.
Thus, a need exists to develop further materials suitable to be used as gas diffusion electrode which overcome at least some of the problems referred to above.